Koroit Opal

Koroit opal is a type of boulder opal, hydrated silica (SiO₂·nH₂O) that forms in veins and pockets within a host of brown ironstone matrix. Koroit opal forms inside nodular ironstone concretions scattered through the Cretaceous sedimentary layers of the Great Artesian Basin in western Queensland. Between roughly 100 and 30 million years ago, silica-rich groundwater percolated through weathered sandstone and mudstone, encountering dense iron-cemented nodules. The silica gel infiltrated the complex fracture networks inside these concretions, then slowly dehydrated and solidified into opal without crystallizing.

The result is an interwoven mosaic of opal veins, seams, and pockets threaded through dark rust-colored ironstone. Unlike the black opal of Lightning Ridge, which forms in flat claystone seams, or pure precious opal from Coober Pedy, Koroit opal keeps its natural ironstone host as a permanent backing. The ironstone provides both structural support for the softer opal and a dramatic dark contrast that amplifies the play-of-color when present.

What makes Koroit distinctive are the 'pictures.' As silica filled irregular cracks in the ironstone, it created abstract compositions that often resemble landscapes, maps, faces, or cellular patterns. Cutters work with the stone rather than against it, shaping cabochons to preserve whatever image the ironstone-opal boundary naturally suggests. This artistic patterning is why Koroit material trades less on pure flash and more on the visual narrative locked inside each nodule.

Genuine Koroit opal shows an unmistakable interplay between dark reddish-brown ironstone and opal veins that range from common potch to vivid precious opal. The boundary between ironstone and opal is always continuous and irregular, never geometric. At 5.5 to 6.5 on the Mohs scale, opal is softer than quartz and scratches with a steel file, while the surrounding ironstone feels notably harder and denser.

Expect specific gravity around 2.0 to 2.5 depending on the ratio of opal to ironstone. Hold a polished piece under angled light and rotate it slowly. Even pieces without strong play-of-color will show subtle internal depth in the opal zones, with faint blue, green, or amber flashes in the thicker seams. The ironstone itself should appear matte to slightly resinous, not glassy. Surface should feel cool and slightly porous along ironstone edges, smooth and glassy across opal zones.